想把Serhii回答的Swift版本加进去。我用下面的代码创建了一个uibareextension .swift:

import Foundation
import UIKit

extension UINavigationBar {
    func hideBottomHairline() {
        self.hairlineImageView?.isHidden = true
    }

    func showBottomHairline() {
        self.hairlineImageView?.isHidden = false
    }
}

extension UIToolbar {
    func hideBottomHairline() {
        self.hairlineImageView?.isHidden = true
    }

    func showBottomHairline() {
        self.hairlineImageView?.isHidden = false
    }
}

extension UIView {
    fileprivate var hairlineImageView: UIImageView? {
        return hairlineImageView(in: self)
    }

    fileprivate func hairlineImageView(in view: UIView) -> UIImageView? {
        if let imageView = view as? UIImageView, imageView.bounds.height <= 1.0 {
            return imageView
        }

        for subview in view.subviews {
            if let imageView = self.hairlineImageView(in: subview) { return imageView }
        }

        return nil
    }
}

Slightly Swift Solution 
func setGlobalAppearanceCharacteristics () {
    let navigationBarAppearace = UINavigationBar.appearance()
    navigationBarAppearace.tintColor = UIColor.white
    navigationBarAppearace.barTintColor = UIColor.blue
    navigationBarAppearace.setBackgroundImage(UIImage(), for: UIBarMetrics.default)
    navigationBarAppearace.shadowImage = UIImage()

}

这里有另一个选择-我认为这只适用于你不需要半透明的导航栏(我没有)。我只是在导航栏的底部添加了一个1像素高的UIView(在导航栏下面1像素)，颜色与我的导航栏相同:

UIView *view = [[UIView alloc] init];
[view setBackgroundColor:self.navigationController.navigationBar.barTintColor];
[self.navigationController.navigationBar addSubview:view];
[view mas_makeConstraints:^(MASConstraintMaker *make) {
    make.height.equalTo(@(1.0f));
    make.leading.trailing.equalTo(self.navigationController.navigationBar);
    make.bottom.equalTo(self.navigationController.navigationBar).offset(1.0f);
}];

我正在使用砌体添加约束。

两条线的解决方案对我来说很有效。尝试在ViewDidLoad方法中添加这个:

navigationController?.navigationBar.setValue(true, forKey: "hidesShadow")
self.extendedLayoutIncludesOpaqueBars = true

在Swift 3中，我们就是这样做的

对于任何视图控制器:

navigationBar.shadowImage = UIImage()
setBackgroundImage(UIImage(), for: .default)

对于整个应用程序:

UINavigationBar.appearance().setBackgroundImage(UIImage(),barMetrics: .Default)
UINavigationBar.appearance().shadowImage = UIImage()

斯威夫特说

UINavigationBar.appearance().setBackgroundImage(UIImage(), forBarPosition: .Any, barMetrics: .Default)
UINavigationBar.appearance().shadowImage = UIImage()

in

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool